A new study shows that cetacean (whale) ancestors probably moved into water before changing their diet (and their teeth) to include carnivory; Indohyus, a 48-million year-old semi-aquatic herbivore, and hippos fall closest to cetaceans when the evolutionary relationships of the larger group are reconstructed.

"Indohyus is interesting because this fossil combines an herbivore's dentition with adaptations such as ear bones that are adapted for hearing under water and are traditionally associated with whales only."

The origin of whales, dolphins, and porpoises—with their highly modified legs and lack of hair—has long been a quandary for mammalogists. Recent genetic research has found that among artiodactyls, hippos are the cetaceans' closest living relatives.

Traditionally, the origin of whales was linked to the mesonychids, an extinct group of carnivores that had singly-hoofed toes. The recent discovery of Indohyus, a clearly water-adapted herbivore, complicates this picture (as new fossils often do) because of ear bones similar to those of modern cetaceans, which are theorized to help the animal have heard better while under the water.

To tease apart different potential evolutionary histories over 80 living and fossil taxa scored for 661 morphological and behavioral characters (such as presence of hair or the shape of and ankle bone). Forty-nine new DNA sequences from five nuclear genes were also added to the mix of more than 47,000 characters.

The team found that the least complex evolutionary tree places Indohyus and similar fossils close to whales, while mesonychids are more distantly related. Hippos remain the closest living relatives.

These results suggest that cetacean ancestors transitioned to water before becoming carnivorous but that the meat-eating diet developed while these ancestors could still walk on land. link